Statistical evaluation of the World Karst Aquifer Map: Global distribution of karst aquifers

The World Karst Aquifer Map (WOKAM), which was completed in 2017, provides the first detailed and consistent global geodatabase concerning the distribution of karstifiable rocks, including carbonate rocks and evaporites, which represent potential karst aquifers and freshwater resources. This study presents the first comprehensive statistical evaluation of WOKAM, focusing on karst in carbonate rocks and addressing four main aspects: (i) Geographic distribution of karst, globally, on all continents, and in the 20 largest countries in terms of surface area and population; (ii) Distribution of karst in different topographic settings, such as lowlands, hills and mountains, and coastal zones; (iii) Occurrence of karst in different climatic zones, ranging from tropical to polar climates and with respect to precipitation regimes; (iv) Estimation of the population living on karst, globally and for individual continents. According to our analysis, 15.2% of the global ice-free continental land surface are characterized by the presence of karstifiable carbonate rock, including both continuous and discontinuous carbonate rock areas. The largest percentage of karst is present in Europe (21.8%), whereas the largest absolute area can be found in Asia (8.35 million km²). China can be described as "karst country number one," with 26.5% of carbonate rock outcrops, corresponding to 2.55 million km². Globally, 31.1% of all carbonate rock areas occur in lowlands, 28.1% in hills, and 40.8% in mountains. About 151,400 km of the world's coastline is characterized by the presence of karstifiable carbonate rocks; the most prominent coastal karst areas include the Dinaric karst, Florida, Yucatan and Southern Australia. Karst occurs in all climatic zones. The highest percentage was identified in temperate climates, where 19.1% of the land surface consist of carbonate rocks, whereas less than 9% of the land surface in tropical and polar climates are occupied by carbonate rocks. Last but not least, we estimated the population living on karst areas on all continents and globally, and how this number has increased in the past 15 years. In 2015, 16.5% of the global population, corresponding to 1.18 billion people, lived on karst areas. These statistical analyses demonstrate quantitatively the importance of karst at different scales, help to raise awareness of karst, and provide a common and defendable basis for discussing global water issues.